The present invention relates to improvements in the field of fuel cells. More particularly, the invention relates to an improved composite used as a bipolar separator plate (BSP) and a process for preparing said composite.
As environmental concerns rise amongst the population, new less polluting energy sources are developed. Proton exchange membrane fuel cells offer an easy way to produce electricity from hydrogen and oxygen with water and heat as by-products. So far, fuel cells have been proposed as an alternative to combustion motors in vehicles as well as for many other applications. A proton exchange membrane (PEM) fuel cell (see FIG. 1) comprises a thin polymer film as electrolyte replacing the liquid electrolyte found in alkaline fuel cells, a cathode on one face of the membrane electrolyte and an anode on the other face of the membrane-electrolyte. In order to increase the fuel cells voltage, these cells are assembled in series. In such a case, a new component, the bipolar separator plate (BSP), is therefore required to separate each cell (see FIG. 1).
A BSP has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. It is imperative that the BSP be as conductive as possible to minimise resistive losses throughout the stack (F. Barbir, J. Baun, J. Neutzler, J. New Mat. Electrochem Systems 2, 1999, 197; R. L. Borup, N. E. Vanderborgh, Mater. Res. Symp. Proc. 1995, 393). Since the BSP also separates the anodic and cathodic compartments, the BSP material should not allow hydrogen or oxygen to permeate it. In a typical stack, the BSP also contains the flow channels for distributing gases on the entire surface of the cell. On top of those properties, BSP materials should be able to survive being assembled to form the fuel cell stack and transported on site. Once in its final form, the BSP should have some basic mechanical strength and be to some degree shock resistant. Furthermore, if the flow channel design is complicated, the material used for making BSP should be easy to machine or be simply processed in its final form, by compression moulding for example.
BSP materials must also be resistant and even practically inert to constant contact with highly acidic environment such as conditions found in PEM fuel cells. The acidity of the membrane (Nafion(copyright)) is roughly equivalent to a solution containing 0.1 M of H+ (S. Gottesfeld, T. A. Zawodzinski, Adv. Electrochem. Sci. and Eng, 5, 1997, 195. ). The pH of water coming out of the anodic and cathodic compartments ranges between 3 and 5. In such acidic conditions, most metals will either form passivating non-conductive oxides or be dissolved like steel. Passivating oxides will decrease the electrical conductivity of the BSP to intolerable levels. On the other hand, ions leached during the dissolution of ferrous materials will contaminate Nafion(copyright) that ultimately leads to poor performance (A. S. Woodman, E. B. Anderson, M. C. Kimble, xe2x80x9cSensitivity of Nafion(copyright) to Metal Contaminants for Proton Conducting Membrane Fuel Cellsxe2x80x9d, The Electrochemistry Society Meeting Abstract, 99-2, 1999). Finally, the material used should also be a good thermal conductor to help redistribution of heat generated inside the stack.
Large scale commercialisation of fuel cells is possible if their production costs are lowered. One of the most expensive components in the proton exchange membrane fuel cell hardware is the BSP. Up to now, the material that has been widely used in making bipolar plates is graphite. Precision machining of these plates is expensive and to ensure that they are impermeable to gases and strong enough, the graphite bipolar plates are rather thick. To replace graphite, the new material must be low cost, easy to shape, light, compact and corrosion resistant. Furthermore, its electrical and thermal conductivity must be high. New processes as well as new materials must therefore be developed to fulfil all these requirements.
Recent Areas of Research on Low Cost BSP Materials and Production Processes
New metallic alloys can be developed to withstand the fuel cell conditions. Also new methods of producing graphite BSP such as injection moulding are being actively pursued. Composites made of metals and graphite are also studied. The latter category encompasses the use of metallic powder in graphite blends that are later processed in many different ways.
New Metallic Alloys
Since the only requirement that most metallic materials fail to meet is chemical stability in an acidic environment, the use of various metals alloys and metallic coatings for making BSP have been studied. There are generally two main approaches pursued to get around the chemical stability problem. First, a noble metallic coating can be applied on a less expensive substrate. The coatings presented by Woodman et al. in xe2x80x9cDevelopment of Corrosion-Resistant Coatings for Fuel Cell Bipolar Plates, Proc. AESF SUR/FIN Annu. Int. Tech. Conf. 1999xe2x80x9d, 717-725, that are gold over aluminium and gold over nickel over copper over aluminium are a good example of this approach. L. Ma et al. in J. New Mat. Electrochem. Systems, 3, 2000, 221, have also studied other coating materials such as TiN. Secondly, existing corrosion resistant alloys have been tested in a fuel cell environment to assess their chemical stability and new metallic alloys have been developed. Austenitic stainless steels containing small amounts of copper like 904 L (N 08904) and N 08926 were investigated by D. P. Davies et al. (J.Power Sources, 86, 2000, 237; J. Appl. Electrochem, 30, 2000, 101) and R. C. Makkus et al. (J.Power Sources, 86, 2000, 274). Also, R. Homung and G. Kappelt (J. Power Sources, 72, 1998, 20) studied a novel iron and nickel-based alloys that appear to be promising.
Even if the use of new metallic alloys and metallic coated alloys appears interesting, there are nonetheless a few unanswered questions. Gold plating complicated patterns on BSP is expensive especially if the coating is similar to the best coating produced by Woodman et al. in xe2x80x9cDevelopment of Corrosion-Resistant Coatings for Fuel Cell Bipolar Plates, Proc. AESF SUR/FIN Annu. Int. Tech. Conf. 1999xe2x80x9d, 717-725 which is gold over nickel over copper over aluminium. New, alloys offer a simple solution to the corrosion problem but they also comprise many major potential problems. Complicated alloys containing more than 50% non-ferrous additives are costly. Furthermore, all of these alloys would produce multivalent cations if dissolved in the fuel cell, causing contamination of the Nafion(copyright) membrane that will cause a decrease in cell performance. Since it is impossible to ensure that a single bipolar plate would not corrode in an entire stack, there will always be a risk when exposed metal is in contact with the electrode.
It is therefore an object of the present invention to provide a composite useful in the fabrication of BSPs. A further object of the present invention is to provide a process for the preparation of the composite.
According to a first aspect of the invention, there is provided a composite comprising:
a steel substrate having a carbon coating thereon, the carbon coating comprising a carbon layer derived by pyrolysis of an acetylenic polymer having a content of carbon of at least 90 weight %, the carbon layer protecting the substrate against corrosion and improving long term stability of contact resistivity of the substrate, the polymer being soluble at a temperature below 110xc2x0 C. in an organic solvent, and the carbon layer is contacting the steel substrate.
Chlorobenzene, chloroform, o-dichlorobenzene, N,N-dimethylacetamide, N,N-dimethylformamide, 1,1,2-trichloroethane, 1,1,1,2-tetrachloroethane 1,1,2,2-tetrachloroethane, tetrahydrofuran, xylene and mixtures thereof, are non-limitative examples of organic solvents that can be effective to dissolve the polymer.
According to a second aspect of the invention, there is provided a process for the preparation of a composite comprising a steel substrate having a carbon coating, comprising the steps of:
a) contacting a solution with a steel substrate and coating a film of the solution on a surface of the steel substrate, the solution comprising an acetylenic polymer and a solvent; and
b) pyrolyzing the film at a temperature ranging from 600 to 1000xc2x0 C. to form the carbon coating comprising a carbon layer.
Applicant has found quite surprisingly that by contacting a solution with a steel substrate and coating a thin film of the solution on a surface of the steel substrate, the solution comprising an acetylenic polymer and a solvent; and pyrolyzing the acetylenic polymer film at a temperature ranging from 600 to 1000xc2x0 C., a low cost composite having a carbon layer protecting the substrate against corrosion and being electrically conducting is obtained.
Steel
In the composite according to the first aspect of the invention, the steel can comprise at least 50 weight % of iron and suitably can be selected from the group consisting of 304, 316 and 316 L stainless steel. Preferably, the stainless steel is 316 L. More preferably, the steel substrate is in the form of a plate.
Acetylenic Polymers: Copolymers and Terpolmers
According to the composite of the first aspect of the invention, the acetylenic polymer can suitably comprise, in a first case up to 85 mole % of m-diethynylbenzene or, in a second case, up to 85 mole % of a mixture of p-diethynylbenzene and m-diethynylbenzene. When a mixture of p-diethynylbenzene and m-diethynylbenzene is used, the mixture can comprises from 0 to 35 mole % of p-diethynylbenzene and preferably from 5 to 10 mole %. According to the first case, the polymer can be a copolymer comprising m-diethynylbenzene and a flexibilizing agent effective to enhance solubility of said polymer in said solvent, below 110xc2x0 C. According to the second case, the polymer can be a terpolymer comprising m-diethynylbenzene, p-diethynylbenzene and a flexibilizing agent effective to enhance solubility of said polymer in said solvent, below 110xc2x0 C. In both cases, the acetylenic polymer preferably comprises from 15 to 30 mole % of the flexibilizing agent, and more preferably 20%. Suitably the flexibilizing agent is an acetylenic monomer and can be for example a monomer of formula: 
Wherein
A is xe2x80x94(CH2)mxe2x80x94 and m has a value of 0, 1 or 2;
E is O or a single bond;
Z is O, S, 
Y is H, CH3 or C6-C12 aryl and n has a value of 0 or 1; and
R and Rxe2x80x2 are xe2x80x94(CH2)pxe2x80x94 or C6-C12 arylene and p has a value of 0, 1 or 2, R and Rxe2x80x2 are the same or different and preferably the same.
Preferably, the C6-C12 aryl is 
and where G is H, CH3, CH2-CH3 or phenyl.
The C6-C12 arylene can be selected from the group consisting of: 
and where G is H, CH3, CH2-CH3 or phenyl.
Preferably, the monomer is selected from the group consisting of 
More preferably, the monomer is 
According to the composite of the first aspect of this invention, the solvent is preferably selected from the group consisting of 1,1,2,2-tetrachioroethane, chlorobenzene, o-dichlorobenzene and mixtures thereof. Preferably, the acetylenic polymer is soluble at a temperature below 80xc2x0 C. More preferably, the solvent is 1,1,2,2-tetrachioroethane and even more preferably, the acetylenic polymer is soluble in a temperature ranging from 50 to 60 xc2x0 C. Finally, the acetylenic polymer has preferably a content of carbon from 92 to 97 weight %.
Coating
The coating, of the composite according to the first aspect of this invention suitably has an electrical resistivity below 0.25 xcexa9-cm. The coating can also comprise an intermediate layer on the carbon layer and an outer layer on the intermediate layer. Preferably, the combination of the carbon layer and the intermediate layer provides a non-porous coating; and the outer layer is effective to protect the intermediate layer. The carbon layer and the outer layer are preferably derived by the pyrolysis of the acetylenic polymer. Also, the intermediate layer preferably comprises pyrolyzed carbon derived by contacting a suspension of particulate carbon in an organic solvent or an aqueous media with the carbon layer to form a film coating, and pyrolising the film coating.
EMI/RFI Shield, Aerodag(copyright) G, Electrodag(copyright) (109B and Electrodag(copyright) 112 are non-limitative examples of suspensions of particulate carbon that can be effective.
More preferably, the suspension comprises particulate carbon selected from the group consisting of carbon black, graphite, acetylene black, Ketjen black and mixtures thereof.
The carbon coating is preferably from 70 to 100 xcexcm thick.
Finally, the coating preferably improves the contact resistance to carbon paper of the substrate.
In the process according to the second aspect of the invention, in step (a) the solution can be sprayed on the surface of the steel substrate to form the film; and the process further comprises after step (a) and prior to step (b):
axe2x80x2) contacting a suspension of particulate carbon in an organic or aqueous solvent with the film to form an intermediate film on the film;
axe2x80x3) contacting the solution with the intermediate film to form an outer film on the intermediate film; and
in step (b), the films are pyrolyzed to form the carbon coating comprising the carbon layer, an intermediate layer and an outer layer.
Preferably, in step (axe2x80x2), the suspension is sprayed on the film to form the intermediate film, and in step (axe2x80x3), the solution is sprayed on the intermediate film to form the outer film. More preferably, in step (a), the solution is sprayed from 10 to 50 times and even more preferably from 15 to 25 times. In step (axe2x80x2), the suspension can be sprayed 1 to 5 times and preferably 2 times. In step (axe2x80x3), the solution can be sprayed from 1 to 5 times and preferably 3 times. Steps (a) and/or (axe2x80x3), can also comprise a spin coating film of the acetylenic polymer prior to a first spray of the solution. Furthermore, steps (a) and/or (axe2x80x3) can comprise cross-linking the acetylenic polymer using UV light. Preferably, steps (a) and/or (axe2x80x3) comprise cross-linking said acetylenic polymer between each spray of said solution using UV light.
Steps (a) and/or (axe2x80x3) can comprise pyrolyzing the film(s) after each spray of the solution. Preferably, Steps (a) and/or (axe2x80x3) comprise pyrolyzing the film(s) after a last spray of a plurality of sprays of the solution. Step (axe2x80x2) can comprise pyrolyzing the film after each spray of a plurality of sprays of the suspension. Preferably, step (axe2x80x2) comprises pyrolyzing the film after a last spray of a plurality of sprays of the suspension.
The solution can have a high concentration of acetylenic polymer ranging from 20 to 200 mg/mL. Preferably, the solution has a low concentration of acetylenic polymer ranging from 10 to 20 mg/mL. The acetylenic polymer is preferably soluble in an organic solvent at a temperature below 110xc2x0 C. The solution can also comprise graphite powder or fiber.
The temperature when pyrolyzing is preferably ranging from 600 to 800xc2x0 C. and more preferably from 700 to 780xc2x0 C. Pyrolyzing preferably occurs under an inert gas atmosphere and more preferably under argon.
Utilisation of the Composite
The composite of the first aspect of the invention can be used in a corrosive environment. More particularly, the composite can be used in a corrosive environment that comprises an acid selected from the group consisting of phosphonic acid, phosphoric acid, sulfonic acid, sulfuric acid and mixtures thereof. The composite is preferably used in a part of fuel cell hardware and more preferably as a bipolar separator plate (BSP). Even more preferably, the composite is used when an electrical conductivity and a corrosion resistance are required. Finally, the composite can be included in a bipolar separation plate of a fuel cell having electrodes and membranes.
Experimental Section
Polymer Synthesis
The synthesis of those polymers with a high carbon content is hereafter exemplified. In 1960 the synthesis of poly (m-diethynylbenzene) 2 (PEB) was obtained by the oxidative polymerisation of m-diethynylbenzene 1 (Hay, A. S. J. Org. Chem. 1960, 25, 1275; Hay, A. S. U.S. Pat. No. 3,300,456, 1967). 
The empirical formula for this polymer is C10H4 and it therefore contains 96.75% of carbon and 3.25% of hydrogen. The polymer can be cast into a transparent film or spun into a fiber. When heated the material begins to cross-link at about 150xc2x0 C. and then converts to glassy carbon at about 600xc2x0 C. General Electric set up a pilot plant for this material which was successfully used for the preparation of high modulus, high strength carbon fibers (Krutchen, C. M., Flom, D. G., Gorowitz, B., and Roberts, B. W. xe2x80x9cLarge Diameter High Strength, High Modulus Carbon Fibers from Polyacetylenesxe2x80x9d in 11th Biennial Conference on Carbon. 1973, Gatlinburg, Tenn.). They also obtained several patents in this field U.S. Pat. No. 3,852,235 in 1974, U.S. Pat. No. 3,899,574 and U.S. Pat. No. 3,928,516 in 1975 and U.S. Pat. No. 3,933,722 in 1976). The conversion to graphite fiber required heating in a RF type of furnace at 2800xc2x0 C. and took place in about 1 minute. Parallel collaborative work at Cosden Petroleum demonstrated that the monomer could be available at reasonable cost. General Electric, however, decided not to commercialise this material.
After the project was terminated very little work on other potential applications was carried out. Films of the material on a substrate could be converted to conducting carbon films and the resistivity obtained varied with the final temperature reached in the heating cycle (Newkirk, A. E., Hay, A. S., and McDonald, R. S., J. Pol. Sci. A, 1964, 2, 2217). Subsequent work by Whitesides demonstrated conclusively that glassy carbon can be obtained at temperatures as low as 600xc2x0 C. (Neenan, T. X., Callstrom, M. R., Scarmoutzos, L. M., Stewart, K. R., Whitesides, G. M., and Howes, V. R., Macromolecules. 1988, 3525-8; Neenan, T. X. and Whitesides, G. M., J. Org. Chem. 1988, 2489; Neenan, T. X., Callstrom, M. R., Bachman, B. J., McCreery, R. L., and Alsmeyer, D. C., Br. Polym. J. 1990. 171). Callstrom later showed that metal nanoparticles could be introduced into the polymers that could be subsequently converted to doped glassy carbons for use as electrodes. 
The parent homopolymer crystallises readily and is not soluble in any solvent at room temperature. Films must be cast from a solvent such as chlorobenzene at  greater than 110xc2x0 C. Copolymers and terpolymers were prepared to break up the regularity of the molecule and therefore increase the solubility. Small amounts of p-diethynylbenzene 3 and the propargyl ether of bisphenol-A 4a were incorporated into the polymer which gave increased solubility at lower temperatures so that the polymer solution could be easily melt extruded (White, D. M., U.S. Pat. No. 3,821,153, 1974). This lowered the carbon content of the polymers by a few per cent but the conversion to graphite was still satisfactory. The homopolymers of a series of dipropargyl ethers that were previously prepared and were very photosensitive and cross-linked readily under ultraviolet light (Hay, A. S., Bolon, D. A., Leimer, K. R., and Clark, R. F J. Pol. Sci. B, 1970. 8, 97; Hay, A. S., Bolon, D. A., and R. Leimer, K. J. Pol. Sci. A-1, 1970 8, 1022). Some work was also done on adding plasticizers, which also had high carbon contents, to the polymer so that moulded structures could be prepared.
For the present applications even greater solubility of the polymer than for those described above was required so that the coating could be applied at low temperatures to simplify the processing. This required the synthesis of further copolymers, terpolymers, with other diethynyl compounds to optimise the solubility and to maintain the high carbon content in the polymers. Larger amounts of dipropargyl compounds, e.g. 4a-c containing large, bulky aromatic groups (Ar) or aliphatic diacetylenes, e.g. 5, were the simplest choice to break up the regularity in the 
structure to give more soluble materials.
In the following tables it is demonstrated that polymers 6 and 7, soluble at room temperature, can be obtained by copolymerizing with about 20 per cent of a dipropargyl derivative of various bisphenols or with an aliphatic diyne. 
The polymers obtained, as shown below in table 1, were all very high molecular weight and could be cast into tough, flexible films.
C is Ar used in M2-48
D is Ar used in M-56
G is Ar used in M2-61
E is Ar used in M2-62
Monomers A to E were synthesized.
Monomers F to H were brought of the shelf.
Preparation of Dipropargyl ether 4c 
A typical example is given as follows. To a dry 100 mL three neck flask equipped with a Dean-Stark trap, water condenser, a magnetic stirrer, and a nitrogen inlet, biphenol 9 (5 g, 14.00 mmol), anhydrous potassium carbonate (4,83 g, mmol), toluene (20 mL) and DMAC (30 mL) were charged. Under an atmosphere of nitrogen, the solution was heated and maintained at 145xc2x0 C. for 2 h to remove all water by means of azeotropic distillation with toluene. The reaction mixture was cooled down to room temperature, and there was added (1.83 mL, 16.80 mmol) of propargyl bromide dropwise over a 10 min period. The reaction mixture was stirred at 70xc2x0 C. for about 2 h. HPLC analysis showed that all the starting material had disappeared. The resulting mixture was cooled and poured into 200 mL of water to precipitate out the product. The product was collected by filtration, and purified by recrystallization three times from ethyl acetate and methanol (3-1) to afford a white powder in 86% yield. Purity: 98.5% (by HPLC) MALDI-TOF-MS: 426.3 (Calc: 426.51). The other dipropargyl ethers which have been reported previously (Hay, A. S., Bolon, D. A., Leimer, K. R., and Clark, R. F. J. Pol. Sci. B, 1970, 8, 97) were prepared in the same manner.
Preparation of Homopolymer 2
This is a modification of previous procedures used for the preparation of this polymer (Hay, A. S. J. Org. Chem. 1960, 25, 1275; Hay, A. S. U.S. Pat. No. 3,300,456, 1967; White, D. M., Hay, A. S, Macromolecular Synthesis, 1973, vii, 11). To a 250 mL wide-mouthed Erlenmeyer flask equipped with an oxygen inlet tube, vibromixer stirrer, and a syringe inlet in an oil-bath at 60xc2x0 C. was added 50 mL of o-dichlorobenzene, 0.3 g of copper (I) chloride, 0.5 mL of N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethlyendiamine and 3 mL of pyridine. Oxygen was bubbled into the solution, which was vigorously stirred. Over a 30 min. period, 4.55 g of m-diethynylbenzene 1 was added via the syringe. The temperature of the reaction mixture rapidly rose to 110xc2x0-112xc2x0 C . After the addition was complete, the reaction was continued for 15 min. The polymer solution was diluted with 50 mL of o-dichlorobenzene and the polymer precipitated into 100 mL of methanol with 10 mL HCl, and than stirred for 1 h. After filtering and drying a pale yellow fibrous polymer obtained in quantitative yield.
Preparation of Polyacetylene 6a Containing 20% BPA Dipropargyl Ether 4a
A mixture of 10 mL of o-dichlorobenzene with dissolved 2,2 bis-(propargyl oxyphenyl) propane (BPA) 4a (1.2175 g, 4 mmol) and m-diethynylbenzene 1 (2.0184 g, 10 mmol) was prepared. This mixture was added dropwise over 30 min to a solution at 50xc2x0 C. of the pre-mixture of 30 mL of o-dichlorobenzene containing copper (I) chloride (0.15 g), N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethlyendiamine 8 drops, and 1.5 ML of pyridine. The temperature increased very slowly from 50xc2x0 C. to 75xc2x0 C. after addition. The reaction was maintained at this temperature for another 10 min. The reaction mixture became very viscous. The polymer solution was diluted with 50 mL of o-dichlorobenzene and the polymer precipitated into 100 mL of methanol, and than stirred for 1 h. After filtering and drying a white fluffy fibrous polymer was obtained. The properties are previously shown in the table. The other copolymers were prepared in the same manner.
Experimental Part
After having reviewed the state of the available technology for making BSP, it appears that none of the proposed solutions was adequate. It was also clear that a new material and a new way of producing BSP were both needed to fulfil the requirements previously stated. The technology proposed herein consists of protecting steel using a carbon coating. This method allows producing a BSP that exploits the bulk mechanical properties of steel while being protected by a carbon layer that is chemically stable. The electrical conductivity of the resulting protected plate should be quite similar to the conductivity of the stainless steel.
To produce carbon coatings, it was proposed that a high carbon content polymer be pyrolyzed on the surface of the steel. A polymer that had been studied for producing carbon fibers in the 1960""s, poly (m-diethynylene benzene), was thought to be a good choice for preparing the coating. When heated at a rate of 1xc2x0 C./min, this polymer produces a black residue with a yield of over 90% (A. E Newkirk, A. S. Hay, R. S McDonald, J. Pol. Sci. A, 1964, 2, 2217; A. S. Hay, U.S. Pat. No. 3,300,456, 1967). This residue carbon content is in excess of 95% (A. S. Hay, J. Org. Chem., 1960, 25, 1275). It was also reported that uniform films could be cast from a poly (m-diethynylene benzene) solution. Combined, these properties would mean that a large amount of polymer would be transformed into carbon, making the production of a pinhole free film possible. Unfortunately, poly (m-diethynylene benzene) is only soluble at high temperature in selected solvents. For example it can be dissolved in o-dichlorobenzene at around 120xc2x0 C. To reduce the complexity of handling this polymer, copolymers containing poly (m-diethynylene benzene) were synthesised. These copolymers are soluble in solvents such as 1,1,2,2 tetrachloroethane and tetrahydrofuran at relatively low temperatures (25xc2x0 C. to 80xc2x0 C.) which makes them easier to work with.